Modern color ink jet printers are often provided with plural printheads, each printhead capable of outputting two or more colors onto a media sheet. In particular, an ink jet printer may be provided with a dual pen arrangement wherein one pen has dark magenta, light magenta and yellow inks and the other pen includes dark cyan, light cyan, and black inks. Hereafter, a printhead with magenta ink will be called the magenta printhead and a printhead with the cyan ink will be called the cyan printhead.
Manufacturing specifications for such printheads set limits for ink drop weights that are emitted from printhead orifices. If the drop weights vary by too great an amount, an unwanted hue-shift occurs in a resulting print.
However, the tighter the limits on the drop weight, the more costly is the manufacturing process for the printhead.
Accordingly, it is desired to provide a means for enabling a printer to accommodate printheads having relatively wide variations in drop weight, while achieving high quality print results.
Manufacturing experience with multicolor printheads indicates that the standard deviation of drop weight between printheads is about three times that of channel-to-channel variations within a single printhead. Accordingly, it is most important to correct for drop weight variations between printheads, rather than attempting to correct for drop weight variations that occur within a single printhead. Further, while minimizing drop weight variance is an important factor in retaining print quality, other factors like media variation, temperature and humidity changes and variations in electronics within the printer also contribute to hue-shifts.
In order to correct for hue-shifts, some high end printers are provided with a sensor, or even a calorimeter, to test the color of certain print tiles. The drop weight of each printhead is then calculated from the calorimetric data and subsequent printing actions are compensated, based on the calculated data. While such a system does not require user input, the disadvantage is the cost of providing a sensor and/or calorimeter within the printer. In many low cost printers, the hue-shift problem is simply ignored.
For printers which produce high quality output prints, the hue shift problem can significantly affect print quality. Statistical analyses shows that 50% of printers will suffer more or less from this problem, and that 5% of those printers will generate prints that are not acceptable to normal users. Attempts have been made to hide the drop weight variation problem rather than to solve it. This is achieved by arranging the printheads so that one is provided with black, cyan-dark and magenta-dark inks, and the other is provided with yellow, cyan-light and magenta-light inks. In such case, no matter how different are the drop weights of the two printheads, dark cyan and dark magenta always track each other in drop weight (because they are emitted from the same printhead). So do the light cyan and light magenta drop weights. As a result, an imbalance between the printheads only has some effect on the image contrast, rather than its hue. Variations in image contrast are more acceptable to most users. The disadvantage is that if there is some misalignment between the two printheads, regions with light ink to dark ink (light magenta to dark magenta, light cyan to dark cyan) will look worse. Further, the dark magenta ink may not overlap properly with the light magenta ink. This will significantly affect certain important color regions, such as flesh tones.
Finally, in order to accommodate the drop weight variation problem, the drop weight of each printhead can be measured on the production line and this information recorded on the printhead and later recognized by the printer. This increases the expense of both printhead production and requires additional cost be added to the printer. Further, the drop weight of each individual printhead can change over the printhead's lifetime.
Accordingly, it is an object of this invention to provide an improved method and apparatus for compensating for print variations which result from variations in drop weight.
It is another object of this invention to provide an improved method and apparatus for color balance calibration of an ink jet printhead which does not require an addition of calibration apparatus to the printer.
It is yet another object of this invention to provide an improved method and apparatus for color balance calibration of an ink jet printer which accommodates for drop weight and media changes over the lifetime of a printhead.